In our European patent specification No 0 289 240 there is disclosed apparatus for forming flakes of material from a heated stream of molten material. The apparatus comprises means for feeding the stream in a downwards direction into a rotating cup, the cup being arranged with its open mouth facing upwardly such that molten material within the cup is caused to flow over the upper edges of the cup and flow outwards in a radial direction due to centrifugal force. The apparatus also includes a pair of spaced apart substantially parallel plates arranged about the cup such that the material leaving the cup by centrifugal force passes through a gap defined between the plates. The plates are mounted within a vacuum chamber arranged such that a vacuum is applied to the space between the plates forming an annular venturi drawing air from outside the chamber between the plates in a radial direction, maintaining angular velocity and preventing the molten material from touching the sides of the plates and cooling the material until reaches a solid state. The material is pulled at velocity by the air stream in an angular direction as the radius expands thereby keeping it in the form of a flat film and ultimately breaking it into small platelets.
It is stated in EP 0 289 240 that by suitable choices of the speed of rotation of the cup, the distance between the two plates and the rate of air flow through the vacuum chamber, the size and thickness of the flakes of material to be produced can be changed and controlled. It is further stated that the apparatus may be used not only with glass but with materials such as basalt, ceramics such as alumina, graphite, and metals such as lead. For each different material it may be necessary to alter the operation of the apparatus by, for instance, varying the speed of rotation of cup, the temperature of the molten material, the size of the gap between the plates and the airflow between the plates. In a further reference to factors which may be varied, reference is made to the volume of molten stream entering the cup, the temperature of that material, the speed of the cup, the diameter of the cup, the distance between the cup and the plates and the airflow at the exit from the vacuum chambers associated with the plates.
As far as concerns the distance between the cup and the plates, there was no clear understanding of the effect of changing this distance. It was known that the distance could only be varied in both directions within certain parameters to produce flake rather than strands or fibres but the main effect was thought to be a reduced thickness of flake with increase in distance between the cup and the plates due to a longer period of stretching the glass before cooling and an increase in nominal flake diameter.
In practice, for some applications glass flakes are required with a particle thickness size distribution as narrow as possible, since this results in a product with the desired range of physical properties for further processing. In other applications a wide spread of particle size distribution may be required, for example to provide optical effects. However, there are, as indicated above, a substantial number of parameters affecting the nature of the product of which the cup plate separation represents only one possible variable. It was thought that the distance between cup and plates should be fixed at a median consistent with the production of acceptable flakes, with the parameters of thickness being changed by varying the plate gap, cup peripheral velocity and air flow. Any change of the cup to plate distance was thought simply to be an additional means of changing the thickness of the product but not of changing the thickness distribution.
Apparatus as described in EP 0 289 240 has been operated for many years on what was thought to be the most optimal combination of operating conditions in order to produce the best product. However, even under such conditions, glass flake is produced with a controlled thickness but quite a wide particle distribution range which was generally thought to be uncontrollable. For many applications, the size range required is outside that produced by such apparatus. Accordingly, the product has to be graded and the material at the bottom and top of the size range has to be removed and then discarded or recycled. This process increases the expense of producing the product and it would be desirable if a product could be produced with a more specific particle size range so that the extent of grading, discarding and recycling of products can be reduced or eliminated.